MINTEMP error, not sure how to fix  

MINTEMP error, not sure how to fix

I'm getting a constant MINTEMP error, and the temperature is consistently reporting 9C lower than the actual temperature.  For reference I've been printing on this printer for months without issue.

Today I had a small blob build up on the hot end from a failed print (both extremely unusual), and I decided to replace the nozzle and heat block which I've done many times before on my 6 printers.  After putting it all back together the nozzle temp will slowly fall to 10C at which point I get the MINTEMP error.  Ambient temp is around 19C, which is what the bed reports.  This did not happen before, both bed and nozzle reported nearly the same temp at ambient.

If I raise the nozzle temp it will report properly and I can actually print normally.  But if it cools down the nozzle temp will keep going lower until the MINTEMP is triggered.

I'm confused by this.  If I damaged the thermistor wires I would not expect the nozzle temp to be reported correctly at higher temps and be able to print normally as well.  

Another thing that may or may not be relevant, I use Octoprint with a RPi4b and after the first MINTEMP error I noticed that when I connected Octoprint to the printer again I could hear the fans spin briefly as it normally does showing a connection, but the temperatures took a long time to be reported in Octoprint.  Normally those show right away upon connection but it took several minutes before those temps were reported.  

Any idea what's going on or how to fix?  Is it worth replacing the thermistor, or is that likely not the problem?